1. Field of the Invention
The present invention relates to a method of enhancing fire resistance of high-strength concrete by mixing a spalling reducer (fiber cocktail) that is a hybrid of a polypropylene fiber and a steel fiber into the concrete and performing proof stress reinforcement (transverse confinement) on main steel bars using a wire rope and spacers.
2. Discussion of Related Art
As the height of buildings increases, the strength of concrete increases. Due to the recent technological development of high-strength concrete, there is increased need of a spalling prevention method as a plan for ensuring fire safety performance of a high-strength concrete column applied to a super high-rise building when a big fire breaks out, as well as a plan for improving load bearing capacity.
The high-strength concrete has characteristics of lower permeability and higher density than ordinary concrete. When exposed to high temperature, the high-strength concrete undergoes spalling due to thermal stress caused by a temperature difference between the interior and exterior of the high-strength concrete and a sharp increase in pore pressure of the interior of the high-strength concrete. For this reason, to ensure structural fire safety of super high-rise buildings to which the high-strength concrete is applied, a composite technique for preventing spalling and simultaneously improving load bearing capacity is required.
In general, to prevent spalling, a sharp rise in temperature should be suppressed, and a moisture content of the concrete should be equal to and less than a range of 5 to 6%. Further, a ratio of water to cement should be maintained at a adequate level. However, these requirements are merely minimum requirements for preventing spalling. To prevent spalling of the high-strength concrete applied to high-rise buildings, additional countermeasures based on a mechanism by which spalling takes place are studied in addition to such basic requirements.
Spalling of concrete advances in a primary spalling process in which the concrete is exposed to high temperature and is accompanied by an exfoliation phenomenon caused within 20 minutes by surface expansion, and secondary and tertiary spalling processes caused by an increase in vapor pressure of pores. Due to a loss of dynamic behavior capability of main bars in the primary spalling process, and a loss of a cross-section in the secondary and tertiary spalling processes, the building may collapse.
Known spalling prevention methods based on a spalling mechanism include a method of applying fireproof paint, a method of using polypropylene and metal lath, a method of applying a fireproof board, and a method of using a mixture of polypropylene and steel fibers.
First, the method of applying fireproof paint has an advantage in that construction is easy while a surface temperature can be controlled, but a disadvantage in that proof stress can be sharply reduced due to separation of a foamable material when a fire breaks out.
Next, the method of using polypropylene and metal lath has an advantage in that scattering caused by spalling can be reduced, but a disadvantage in that securing of concrete fluidity (coagulation of polypropylene that is organic fiber) and quality control are difficult.
Here, polypropylene (a kind of organic fiber, and called PP fiber) is melted when internal temperature of concrete continues to increase during a fire, thereby forming a microstructure serving as a passage through which vapor can be discharged. The metal lath is a kind of metal mesh that is installed for traverse confinement of steel bars arranged in the concrete.
Next, the method of applying a fireproof board has an advantage in that heated temperature can be controlled, but a disadvantage in that an effective space is reduced due to an increase in cross-section of a member.
The method of using a mixture of polypropylene and steel fibers has an advantage in that compressive strength is increased due to an increase in tensile strength due to the steel fiber, but a disadvantage in that securing of fluidity and quality control are also difficult.
In addition, the higher the strength of high-strength concrete, the smaller the size of a pore contributing to discharge of vapor. There is a disadvantage in that there is a limit in preventing spalling based on a method of mixing in the organic fiber.
Further, the existing method of applying fireproof paint mentioned above emphasizes prevention of spalling, but both the prevention of spalling and securing of load bearing capacity are required because actual columns are load-bearing members.
Particularly, the fiber (e.g. polypropylene fiber) mixing method has an advantage in that spalling can be prevented by facilitating discharge of vapor through pores secured by a low melting point in the event of a fire, but a problem in that the load bearing capacity can be significantly reduced by pores generated when the fiber melts in a fire.
Therefore, when the fiber mixing construction method is applied as a method of controlling spalling of a high-strength concrete column, a method capable of securing the load bearing capacity should always be applied. Otherwise, fire resistance of the high-strength concrete column applied to super high-rise buildings cannot be secured.
Here, the conventional metal lath is used as a means for confining steel bars. However, there is a limit in securing the load bearing capacity of the high-strength concrete column using only the metal lath such as the steel mesh. Further, there are problems in that constructability is low and there is no alternative but to be limited to concrete pouring in the long run.
FIG. 1a shows an example of a conventional column structure reinforced by wire ropes 20.
That is, main steel bars 10 are arranged inside a column structure in a vertical direction. In place of conventional shear steel bars (transverse reinforcing bars or stirrups), wire ropes 20 wind the main steel bars 10, and opposite ends thereof are connected by fasteners 30. Spacers 40 are supported inside the main steel bars 10.
However, the wire ropes 20 are vertically spaced apart from each other around the main steel bars one by one. Consequently, it can be found that one shear steel bar is reinforced by one wire rope 20. In this main steel bar reinforcement, the wire rope arranging method for securing fire resistance is not separately disclosed.
FIG. 1b shows an example of conventional construction of a spiral shear reinforcement (formed of a reinforcing fiber and a resin) 50. It can be found that this spiral shear reinforcement is continuously disposed along main steel bars in a spiral pattern in a vertical direction, but the wire rope arranging method for securing fire resistance is not separately disclosed.